The Jiashi copper deposit is a typical representative of sandstone-type copper deposits in the Keping basin. Many geologists have studied its genesis, but have no detailed geochemical evidence. According to research findings of this paper, the metallogenic period of the deposit can be divided into diagenetic metallogenic period and reformation metallogenic period, with the former being the main stage. In this paper, the relationship between the ore-forming fluids evolution and mineralization was investigated by means of microthermometry, composition analysis, hydrogen, oxygen, and carbon and sulfur isotopes of fluid inclusions in the two metallogenic periods of the deposit. According to the results obtained, the ore-forming fluids in the diagenetic period had the homogeneous temperature 131~206℃, salinity 0.35%~9.86%, density 0.90~0.99 g/cm³, and pressure 227~464 MPa; in the reformation period, the homogeneous temperature was 131~285℃, salinity was 0.53%~9.34%, density was 0.78~1.00 g/cm³, and pressure was 231~448 MPa. The ore-forming fluid was of medium low-temperature, medium-pressure, low-salinity and medium density type, and it had typical characteristics of underground hot brine. The δ³⁴S values range from -34.5 ‰ to -22.7 ‰, indicating that the sulfur of the deposit was mainly derived from sulfate bacteria reduction and organic matter reduction. The δD values of inclusions of diagenetic period are -105.4‰~-79.2‰, δ¹⁸O_H2O are -3.13‰~0.87‰; in the reformation period the δD values are -108.5‰~-81.3‰, and δ¹⁸O_H2O are -4.21‰~4.04‰, indicating that the ore-forming fluid was the mixed source of atmospheric precipitation and basin brine, with the addition of organic water. The δ¹³C values at the diagenetic stage range from -28.4 ‰ to -24.2‰, and during the reformation period the values range from -28.5‰ to -23.0‰, in agreement with the negative range characteristics of natural organic carbon δ¹³C. Laser Raman probe composition analysis shows that the main components of the ore-forming fluid of the deposit during the diagenetic period and reformation period were mainly CH₄, H₂S and H₂O. It is shown that the organic matter in the oilfield brine was involved in the metallogenic process during the diagenetic period and the reformation period, and played an important role of reduction in the ore-forming process of the Jiashi copper deposit. This study shows that organic matter was closely related to the mineralization of the Jiashi copper deposit, which was a sedimentation-reformation type sandstone copper deposit in which the oilfield brine took part in the mineralization.